The present invention relates to an arrangement of a sensor, in particular an ultrasound sensor for process control of a welding process.
More particularly, the present invention relates to a connecting element for connection of an electrode with an electrode arm of a holder device for holding welding electrodes.
The present invention also relates to a holding device, in particular of a welding apparatus for resistance welding.
For process control of a welding process, in particular for process control of a welding process with resistance point welding, it has been found very efficient to treat the region to be welded with ultrasound during the welding process and to use the ultrasound absorption rate or the residual sound level which remains after the ultrasonic treatment as control variables for the process control. The basic physical process for this purpose is disclosed in the international patent application WO 94/03799. As described there, shear or transverse waves are passed through the welding region or the joining parts during the welding process. For this purpose on the electrode shaft of the first of both oppositely located welding electrodes, an ultrasound transmitter is mounted for producing ultrasound waves and for directing these sound waves in the electrode shaft in direction to the welding region. On the electrode shaft of the second welding electrode which is located opposite to the first welding electrode, a receiver is arranged for receiving the ultrasound which passes through the welding region. The generated ultrasound signal passes from the ultrasound transmitter as structure-born sound through the electrode shaft of the first electrode to a welding product, for example two metal sheets to be welded with one another, and passes through it, so as to finally reach the ultrasound receiver through the shaft of the second electrode. The received sound level is converted there into an electrical signal, and supplied as control variables to the control device as input variables, for controlling the value of the welding current, with which the welding electrodes are provided.
It has been determined that the use of transverse ultrasound waves or ultrasound waves with high transverse portion is especially well suitable for evaluation of the welding condition and for control of the welding process. Transverse ultrasound waves during passage through a fluid lens which is produced during welding as a welding lens of the molten metal, are evaporated especially intensely. Thereby, based on the measurements of the evaporation of the transmitted ultrasound intensity, a reliable expression of the formation and size of the welding lens is possible, which can be used as control variables in for a control process for controlling the welding process.
An important criterium for a good performance of the method, in addition to the use of suitable ultrasound waves, is in particular the type of the introduction of the sound into the electrodes, the sound propagation in the electrodes, and finally the reception of the sound. In the international patent document WO 94/03799 the ultrasound transmitter and the ultrasound receiver for this purpose are mounted on the electrode shafts of the electrodes or on the electrode holders. The ultrasound transmitter and receiver are here shear waves testing heads which have piezo elements. During excitation with an electrical voltage it carries out a shear movement, and vice versa during excitation with a shear movement it generates a receiving voltage.
For receiving a high radiation of the ultrasound waves in direction of the welding product as well also an oriented reception of the sound waves, it is proposed in the document WO 94/03799 to use attachment wedges under the transmitters and receivers. Thereby an orientation of the sound radiation is produced in direction of the central axis of the electrode.
The arrangement disclosed in the international patent document WO 94/03799 has however the disadvantage that the concept of arranging the ultrasound transmitter and the ultrasound receiver is expensive to develop and to realize. For each use, electrode holders or electrodes provided with transmitters or receivers must be manufactured.
German patent document DE 199 37 479 A1 discloses a sensor device, in which the sensor carrier has a receptacle for receiving the transmitter or receiver and thereby a direction-improved sound introduction or a direction-improved sound reception is possible. In the same way as above, each sensor carrier must be provided however with a transmitter and/or a receiver.
In order to make possible welding of a plurality of components with different shapes, conventionally a plurality of electrodes of different lengths, diameters and shapes as well as a plurality of different electrode holders are utilized. In order to realize the solution proposed in the German document DE 199 37 479 A1, it is necessary to provide a plurality of electrodes and/or electrode holders with transmitters and receivers and keep their supply. When the transmitter and receiver are applied in the electrode shaft, it is possible that the commercial electrode shafts which are producible as simple rotary parts in a cost favorable manner, can no longer be used. In case of a damage of the electrode shaft or of the sensor, in addition the whole unit including the electrode shaft together with the sensor must be exchanged.
In an unpublished German patent application of the applicant it is proposed to arrange the transmitter and receiver in a ring-shaped, longitudinally slotted clamping element, such that the clamping element can be fitted on the cylindrical electrode shaft of an electrode and fixed by clamping on the longitudinal slot on the electrode. In this way it is possible to exchange for example the electrodes, but to use the transmitter and receiver further by simply mounting them on a new electrode.
It has been however determined that mounting of a clamping element on the shaft of a welding electrode is often not possible due to narrow available space or the welding would be significantly hindered.
In FIGS. 2 and 3 welding situations are shown, in which the distance to one component to be welded, or to a clamping element which holds the component to be welded, is very small. The additional clamping element proposed in this unpublished patent application therefore can not be used.